3,365 research outputs found
ASHEE: a compressible, equilibrium-Eulerian model for volcanic ash plumes
A new fluid-dynamic model is developed to numerically simulate the
non-equilibrium dynamics of polydisperse gas-particle mixtures forming volcanic
plumes. Starting from the three-dimensional N-phase Eulerian transport
equations for a mixture of gases and solid particles, we adopt an asymptotic
expansion strategy to derive a compressible version of the first-order
non-equilibrium model, valid for low concentration regimes and small particles
Stokes . When the model reduces to the dusty-gas one. The
new model is significantly faster than the Eulerian model while retaining the
capability to describe gas-particle non-equilibrium. Direct numerical
simulation accurately reproduce the dynamics of isotropic turbulence in
subsonic regime. For gas-particle mixtures, it describes the main features of
density fluctuations and the preferential concentration of particles by
turbulence, verifying the model reliability and suitability for the simulation
of high-Reynolds number and high-temperature regimes. On the other hand,
Large-Eddy Numerical Simulations of forced plumes are able to reproduce their
observed averaged and instantaneous properties. The self-similar radial profile
and the development of large-scale structures are reproduced, including the
rate of entrainment of atmospheric air. Application to the Large-Eddy
Simulation of the injection of the eruptive mixture in a stratified atmosphere
describes some of important features of turbulent volcanic plumes, including
air entrainment, buoyancy reversal, and maximum plume height. Coarse particles
partially decouple from the gas within eddies, modifying the turbulent
structure, and preferentially concentrate at the eddy periphery, eventually
being lost from the plume margins due to the gravity. By these mechanisms,
gas-particle non-equilibrium is able to influence the large-scale behavior of
volcanic plumes.Comment: 29 pages, 22 figure
Ash plume properties retrieved from infrared images: a forward and inverse modeling approach
We present a coupled fluid-dynamic and electromagnetic model for volcanic ash
plumes. In a forward approach, the model is able to simulate the plume dynamics
from prescribed input flow conditions and generate the corresponding synthetic
thermal infrared (TIR) image, allowing a comparison with field-based
observations. An inversion procedure is then developed to retrieve ash plume
properties from TIR images.
The adopted fluid-dynamic model is based on a one-dimensional, stationary
description of a self-similar (top-hat) turbulent plume, for which an
asymptotic analytical solution is obtained. The electromagnetic
emission/absorption model is based on the Schwarzschild's equation and on Mie's
theory for disperse particles, assuming that particles are coarser than the
radiation wavelength and neglecting scattering. [...]
Application of the inversion procedure to an ash plume at Santiaguito volcano
(Guatemala) has allowed us to retrieve the main plume input parameters, namely
the initial radius , velocity , temperature , gas mass ratio
, entrainment coefficient and their related uncertainty. Moreover,
coupling with the electromagnetic model, we have been able to obtain a reliable
estimate of the equivalent Sauter diameter of the total particle size
distribution.
The presented method is general and, in principle, can be applied to the
spatial distribution of particle concentration and temperature obtained by any
fluid-dynamic model, either integral or multidimensional, stationary or
time-dependent, single or multiphase. The method discussed here is fast and
robust, thus indicating potential for applications to real-time estimation of
ash mass flux and particle size distribution, which is crucial for model-based
forecasts of the volcanic ash dispersal process.Comment: 41 pages, 13 figures, submitted pape
Modeling dispersed gas-particle turbulence in volcanic ash plumes
This PhD thesis focuses on numerical and analytical methods for simulating the dynamics of volcanic ash plumes. The study starts from the fundamental balance laws for a multiphase gas\u2013 particle mixture, reviewing the existing models and developing a new set of Partial Di\ufb00erential Equations (PDEs), well suited for modeling multiphase dispersed turbulence. In particular, a new model generalizing the equilibrium\u2013Eulerian model to two-way coupled compressible \ufb02ows is developed. The PDEs associated to the four-way Eulerian-Eulerian model is studied, investigating the existence of weak solutions ful\ufb01lling the energy inequalities of the PDEs. In particular, the convergence of sequences of smooth solutions to such a set of weak solutions is showed. Having explored the well-posedness of multiphase systems, the three-dimensional compressible equilibrium\u2013Eulerian model is discretized and numerically solved by using the OpenFOAM\uae numerical infrastructure. The new solver is called ASHEE, and it is veri\ufb01ed and validated against a number of well understood benchmarks and experiments. It demonstrates to be capable to capture the key phenomena involved in the dynamics of volcanic ash plumes. Those are: turbulence, mixing, heat transfer, compressibility, preferential concentration of particles, plume entrainment. The numerical solver is tested by taking advantage of the newest High Performance Computing infrastructure currently available. Thus, ASHEE is used to simulate two volcanic plumes in realistic volcanological conditions. The in\ufb02uence of model con\ufb01guration on the numerical solution is analyzed. In particular, a parametric analysis is performed, based on: 1) the kinematic decoupling model; 2) the subgrid scale model for turbulence; 3) the discretization resolution. In a one-dimensional and steady-state approximation, the multiphase \ufb02ow model is used to derive a model for volcanic plumes in a calm, strati\ufb01ed atmosphere. The corresponding Ordinary Di\ufb00erential Equations (ODEs) are written in a compact, dimensionless formulation. The six non-dimensional parameters characterizing a multiphase plume are then written. The ODEs is studied both numerically and analytically. Di\ufb00erent regimes are analyzed, extracting the \ufb01rst integral of motion and asymptotic solutions. An asymptotic analytical solution approximating the model in the general regime is derived and compared with numerical results. Such a solution is coupled with an electromagnetic model providing the infrared intensity emitted by a volcanic ash plume. Key vent parameters are then retrieved by means of inversion techniques applied to infrared images measured during a real volcanic eruption
DNS of compressible multiphase flows through the Eulerian approach
In this paper we present three multiphase flow models suitable for the study
of the dynamics of compressible dispersed multiphase flows. We adopt the
Eulerian approach because we focus our attention to dispersed (concentration
smaller than 0.001) and small particles (the Stokes number has to be smaller
than 0.2). We apply these models to the compressible ()
homogeneous and isotropic decaying turbulence inside a periodic
three-dimensional box ( cells) using a numerical solver based on the
OpenFOAM C++ libraries. In order to validate our simulations in the
single-phase case we compare the energy spectrum obtained with our code with
the one computed by an eighth order scheme getting a very good result (the
relative error is very small ). Moving to the bi-phase case,
initially we insert inside the box an homogeneous distribution of particles
leaving unchanged the initial velocity field. Because of the centrifugal force,
turbulence induce particle preferential concentration and we study the
evolution of the solid-phase density. Moreover, we do an {\em a-priori} test on
the new sub-grid term of the multiphase equations comparing them with the
standard sub-grid scale term of the Navier-Stokes equations.Comment: 10 pages, 5 figures, preprint. Direct and Large Eddy Simulations 9,
201
Performance of CMS muon reconstruction in pp collision events at sqrt(s) = 7 TeV
The performance of muon reconstruction, identification, and triggering in CMS
has been studied using 40 inverse picobarns of data collected in pp collisions
at sqrt(s) = 7 TeV at the LHC in 2010. A few benchmark sets of selection
criteria covering a wide range of physics analysis needs have been examined.
For all considered selections, the efficiency to reconstruct and identify a
muon with a transverse momentum pT larger than a few GeV is above 95% over the
whole region of pseudorapidity covered by the CMS muon system, abs(eta) < 2.4,
while the probability to misidentify a hadron as a muon is well below 1%. The
efficiency to trigger on single muons with pT above a few GeV is higher than
90% over the full eta range, and typically substantially better. The overall
momentum scale is measured to a precision of 0.2% with muons from Z decays. The
transverse momentum resolution varies from 1% to 6% depending on pseudorapidity
for muons with pT below 100 GeV and, using cosmic rays, it is shown to be
better than 10% in the central region up to pT = 1 TeV. Observed distributions
of all quantities are well reproduced by the Monte Carlo simulation.Comment: Replaced with published version. Added journal reference and DO
Performance of CMS muon reconstruction in pp collision events at sqrt(s) = 7 TeV
The performance of muon reconstruction, identification, and triggering in CMS
has been studied using 40 inverse picobarns of data collected in pp collisions
at sqrt(s) = 7 TeV at the LHC in 2010. A few benchmark sets of selection
criteria covering a wide range of physics analysis needs have been examined.
For all considered selections, the efficiency to reconstruct and identify a
muon with a transverse momentum pT larger than a few GeV is above 95% over the
whole region of pseudorapidity covered by the CMS muon system, abs(eta) < 2.4,
while the probability to misidentify a hadron as a muon is well below 1%. The
efficiency to trigger on single muons with pT above a few GeV is higher than
90% over the full eta range, and typically substantially better. The overall
momentum scale is measured to a precision of 0.2% with muons from Z decays. The
transverse momentum resolution varies from 1% to 6% depending on pseudorapidity
for muons with pT below 100 GeV and, using cosmic rays, it is shown to be
better than 10% in the central region up to pT = 1 TeV. Observed distributions
of all quantities are well reproduced by the Monte Carlo simulation.Comment: Replaced with published version. Added journal reference and DO
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